Various types of catalysts have been proposed for hydrotreating hydrocarbon oils. The so-called two-element catalysts, with the Group 6 elements (e.g., molybdenum and tungsten) and Group 8 elements (e.g., cobalt and nickel) as the active metallic components carried by refractory inorganic oxides (e.g., alumina, silica and magnesia), have been already commercialized. These catalysts have been further developed to have higher desulfurization and/or denitrogenation activity, both from active metallic components and carriers. The applicant of the present invention have already studied to further improve catalyst activity by improving dispersibility of the active metallic components, to propose an extremely high-activity catalyst with high desulfurization activity, which is prepared by supporting cobalt and/or nickel as the Group 8 metals on a silica-alumina carrier in the first step, and further supporting molybdenum and/or tungsten as the Group 6 metals on the same carrier in the second step, to finely disperse molybdenum as the major component on the carrier (Japanese Laid-open Patent application No. 225645/1985).
The carriers have been also developed, by controlling pore size distributions of silica-alumina carriers, to improve desulfurization activity of the catalysts for hydrotreating by maximizing the pores having a diameter of 30 .ANG. to 100 .ANG..
Recently, however, reduction of sulfur content of gas oils is strongly required for environmental reasons, especially for stocks of higher sulfur contents, e.g., light gas oil (LGO) and vacuum gas oil (VGO). In particular, sulfur content of LGO is strongly required to be reduced to 0.05 wt. % or lower for environmental reasons. Whether this is achieved or not largely depends on whether sulfur compounds difficult to remove, e.g., 4-methyl dibenzothiophene and 4,6-dimethyl dibenzothiophene, are efficiently desulfurized, in particular at a high hydrogen sulfide partial pressure.
It is however known that the two-element catalysts are rapidly deactivated, when deeply hydrotreating hydrocarbon oils of high sulfur content, as a result of increased hydrogen sulfide partial pressure in the reaction atmosphere. In particular, the Ni--Mo catalyst, although showing a high desulfurization activity at a low hydrogen sulfide partial pressure, is rapidly deactivated at a high hydrogen sulfide partial pressure, because of its insufficient tolerance to the inhibiting effects by hydrogen sulfide. On the other hand, the Co--Mo catalyst, although higher in tolerance to hydrogen sulfide to some extent, has a disadvantage of lower desulfurization activity. It is therefore necessary to develop a catalyst simultaneously showing a high desulfurization activity and tolerance to the inhibiting effects by hydrogen sulfide, in order to deeply desulfurize hydrocarbon oils.
A variety of techniques have been proposed to solve these problems, viewed from carrier types, carrier structures, active metal components and method for supporting active metals on the carriers. For example, Japanese Laid-open Patent application No. 164334/1997 discloses the hydrotreating catalyst to desulfurize the difficult-to remove sulfur compounds present in gas oil, where an inorganic oxide carrier supports 5 mass % to 20 mass % (as oxide, percentage being based on the catalyst) of molybdenum in the first stage, which is dried and calcined, and then with 5 mass % to 15 mass % (as oxide) of molybdenum and 1 mass % to 10 mass % (as oxide) of nickel in the second stage, which is dried and calcined at 150.degree. C. to 350.degree. C. This catalyst, however, is an insufficient one for the catalyst for deep desulfurization of hydrocarbon oils, because of its low tolerance to the inhibiting effects by hydrogen sulfide.